The field of the invention is improved silver catalysts for the production of ethylene oxide, their preparation and their use in ethylene oxide processes.
The state of the art of processes for preparing ethylene oxide by the catalytic vapor phase oxidation of ethylene with molecular oxygen in the presence of supported silver catalysts may be ascertained by reference to U.S. Pat. Nos. 4,012,425 and 4,039,561, the disclosures of which are incorporated herein. U.S. Pat. Nos. 3,793,231; 3,962,136 and 4,066,575 disclose the state of the art of silver catalysts for the production of ethylene oxide, the disclosures of which are incorporated herein. The reactivation of silver catalysts is disclosed in U.S. Pat. Nos. 4,051,068 and 4,125,480, the disclosures of which are incorporated herein.
U.S. Pat. No. 4,012,425 discloses a process for the production of ethylene oxide wherein ethylene is contacted in vapor phase with an oxygen-containing gas at ethylene oxide forming conditions at an elevated temperature of from 210.degree. to about 285.degree. C. in the presence of a fixed bed of a silver metal-containing catalyst. The catalyst is prepared by
(a) impregnating a porous refractory catalyst support with a solution comprising solvent, silver salt sufficient to deposit from 2% by weight to 20% by weight of silver on the support and salts of one or more higher alkali metals in which the salts consist of salts of cesium or rubidium or mixtures thereof sufficient to deposit an amount of alkali metal in excess of 8.0.times.10.sup.-3 gew per kilogram of total catalyst on the support;
(b) separating the impregnated support;
(c) reducing the silver salt present on the impregnated support to silver metal at a temperature of from about 100.degree. to about 500.degree. C.;
(d) contacting the impregnated support wherein silver is present as silver metal one or more times with an alkanol of 1 or 2 carbon atoms to selectively remove an amount of higher alkali metal present on the impregnated support such that the impregnated support so contacted contains 4.0.times.10.sup.-5 to 8.0.times.10.sup.-3 gew of higher alkali metal per kilogram of catalyst present in final form on the support in the form of an oxide in which the oxide consists of oxides of cesium or rubidium or mixtures thereof; and
(e) separating the impregnated support from the alkanol contacting solution and drying to substantially remove the residual alkanol present in and on the impregnated support.
Supported silver catalysts are used to prepare ethylene oxide by oxidizing ethylene with oxygen or oxygenated gases. It is also known to react these silver catalysts with so-called promoters, the earth-alkali-metal compounds. Barium compounds and/or alkali-metal compounds, especially those of the so-called heavy alkali-metals, rubidium and/or cesium are used as the promoters as disclosed in U.S. Pat. Nos. 3,962,136; 4,066,575 and 4,039,561. Ordinarily, the promoter metal compounds are deposited simultaneously with the silver during the preparation of the catalyst. The promoter metal compounds may also be deposited on the carrier after the silver has been deposited.
It is further known that silver catalysts lose selectivity in the course of time and that after several years' use the catalysts must be replaced by new catalysts. Exchanging catalysts which are degraded in performance by new ones is time consuming and labor intensive in large scale industrial plants. Furthermore, the change-over requires a production stoppage and high costs.
It is also known to improve the performance of a silver catalyst, i.e., to reactivate a used catalyst as disclosed in U.S. Pat. Nos. 4,051,068 and 4,125,480.
This reactivation is implemented for instance in large scale industrial plants by flooding the reactor filled with the catalyst with a solution of the promoter metal compound. After the excess solvent is removed, the residual solvent is eliminated by heating the catalytic bed and blowing an inert gas therethrough. This procedure requires the use of solvents and again causes a production stoppage.